Wa'alaikumassalam sis! Eh, common je tu dalam trimester 3, a few of my pregnant friends complained pasal mulut masam jugak, especially towards the end. Boleh jadi hormone changes or maybe your stomach acid acting up sebab the baby is pushing everything around at this stage. Try minum air more, makan frequent small meals instead of big ones, dan kurangkan spicy or acidic food for now. Tapi kalau it's really bothering you or pasal lain, better check with your doctor jadi they can make sure everything is okay with the baby, takut kalau ada other symptoms pulak kan.